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From seabirds to Arctic mammals, chimpanzees to orangutans, certain species appear to kiss. Currently, scientists propose that Neanderthals did it too – and possibly exchanged kisses with early Homo sapiens.

Shared Oral Clues

This isn't the initial instance scientists have suggested ancient relatives and early modern humans were closely connected. In previous studies, researchers have found humans and their thick-browed cousins possessed the identical oral bacteria for hundreds of thousands of years after the evolutionary divergence, suggesting they swapped saliva.

"Probably they were kissing," she said, adding that the idea chimed with studies that has revealed humans of certain genetic backgrounds contain Neanderthal DNA in their genome, revealing genetic mixing was occurring.

Intimate Spin

"This offers a different spin on human-Neanderthal relations," the lead researcher said.

Publishing in the publication Evolution and Human Behavior, the researcher and colleagues report how, to explore the evolutionary origins of kissing, they first had to come up with a description that was not limited to how people smooch.

Defining Intimate Contact

"Previously there were some efforts to describe a kiss, but it's largely focused on humans, which implies that essentially other animals do not engage in this. Now we know that they likely engage, it might just not look from what our intimate contact resembles," explained Brindle.

However, she noted some behaviors that resembled kissing were distinct activities – such as the processing and food sharing, or "mouth contact", observed in fish known as French grunts.

As a result the research group developed a definition of intimate contact based on social behaviors involving directed oral interaction with a member of the same species, with some movement of the oral area but no transfer of food.

Study Methods

The lead researcher explained they focused on accounts of kissing in primates from the African continent and Asian regions, including primates, apes and orangutans, and employed digital recordings to verify the reports.

Scientists then integrated this information with details on the evolutionary relationships between extant and extinct types of such primates.

Evolutionary Origins

The team say the findings indicate kissing evolved approximately 21.5m and 16.9m years ago in the predecessors of the large apes.

The position of Neanderthals on this evolutionary lineage means it is probable they, too, indulged in a intimate act, the researchers say. But the behavior might not have been limited to their specific group.

"Reality that modern people engage intimately, the reality that we currently have demonstrated that ancient relatives probably kissed, suggests that the two [species] are also likely to have kissed," the researcher noted.

Biological Importance

While the scientific reasoning is debated, Brindle said kissing could be employed in sexual contexts to potentially enhance reproductive success or help choose between partners, while it could assist reinforce bonding when practiced in a platonic way.

Another expert in the behavior of primates commented that as intimate contact was seen in a wide range of apes it was logical its origins extend far into our evolutionary past, and an analysis of different forms of intimate behavior among a broader range of species might extend its origins back even earlier still.

"Behaviors that we think of as signatures of human life, like kissing, are not exclusive to us if we look closely at other animals," the expert noted.

Cultural Elements

An archaeology expert explained that kissing had a cultural element as it was not common to all human groups.

"Nonetheless, as humans we thrive or fail on the strength of our relationships, and ways of encouraging confidence and closeness will have been significant for millions of years," she said. "It might be an concept that seems a bit incongruous to our misplaced ideas of a rather ruthless and aggressive past, but actually it should be expected that ancient hominins – and including Neanderthals and our own species collectively – engaged intimately."